Competing Interests

Applied Biosystems (AB) provided reagents for genotyping of GSTM1 for this study. Toinette Hartshorne and Kelly Li are employed by AB. Neither AB nor any of its employees had a role in study design, data collection and analysis, decision to publish. Toinette Hartshorne and Kelly Li were involved in preparation of the manuscripts and are co-authors for this paper. This does not alter the authors' adherence to all the PLoS ONE policies on sharing data and materials.